A (1) Field of the Invention
The invention relates to a terminal arrangement for a duplex transmission system for digital signals, comprising a transmitting arrangement and a receiving arrangement and a coupling arrangement for coupling the transmitting and receiving arrangements to a duplex transmission path, the receiving arrangement comprising an equalizer for reception and the transmitting arrangement comprising an equalizer for transmission and means being provided for adjusting the equalizer for transmission in response to the received digital signals in combination with the equalizer for reception.
The terminal arrangement is, for example, intended for a digital subscriber connection in a two-wire full-duplex system, which has for its object to replace and improve existing analog subscriber connections, the existing subscribers' lines being utilised.
The frequency band required for such a system may be between approximately 0, 1-200 kHz.
When equipment for the system is being installed, only a few details will be known of the subscribers' lines, such as, for example: length, diameter, cross-talk signals, interferences and the number of stubs, and equalization will be required which can automatically adapt itself for an optimum reception.
An adaptive quantized feedback equalizer (AQF-equalizer) satisfies this requirement. This equalizer adapts itself continuously to a maximum eye opening of the digital signal.
In principle such an equalizer would be required at the subscriber set as well as in the exchange.
In order to keep the dissipation in the subscriber set as low as possible, and to prevent the subscriber set from being unnecessarily expensive, methods to have the pulse correction be effected in the exchange only are sought for.
A (2) Description of the Prior Art
Generally, cable losses are corrected by means of an equalizing network or a pulse-shaping method. Equalization may, in principle, be effected at the beginning or at the end of a transmission path, while pulse-shaping is usually effected afterwards, that is to say at the receiving side.
In its most simple form an equalizing network is an RC-circuit with which the attenuation variation of the cable is indeed equalized, but an unwanted phase shift is introduced at the same time. As a result thereof, the correction of digital signals will not be optimal.
The cross-talk signals from adjacent wire pairs in a cable are the stronger according as the frequency of the noise signals becomes higher. Equalization by means of a network now has the effect that at high frequencies these noise signals will be still more amplified, relatively so that the signal-to-noise ratio is considerably affected.
A great disadvantage of this equalization is therefore the decrease in the signal-to-noise ratio, as a result of which the maximum useful cable length decreases.
For digital signals the quality of the transmission is determined by the number of bit errors produced during regeneration. This number of errors is determined by the extent to which the so-called eye is still sufficiently open.
In order to ensure that the intersymbol interference is as low as possible, it is a requirement that at the sampling moments which are located at n-times the bit period (t=n .tau.), the signal obtained in response to the preceding data pulses must be as small as possible.
The equalizer for transmission provides that intersymbol interference at the receiving end is as small as possible. This method is used at the transmitter instead of at the receiver.
A terminal arrangement of the type described in A(1) is disclosed in Japanese Pat. No. 884,474, issued Sept. 30, 1977, in which the equalization for the return direction is derived from the receiving arrangement, it being assumed that the return line is equal to the receiving line.